/*
 *  linux/kernel/vsprintf.c
 *
 *  (C) 1991  Linus Torvalds
 */

/* vsprintf.c -- Lars Wirzenius & Linus Torvalds. */
/*
 * Wirzenius wrote this portably, Torvalds fucked it up :-)
 */

#include <stdarg.h>
#include <string.h>

/* we use this so that we can do without the ctype library */
#define is_digit(c)	((c) >= '0' && (c) <= '9')
//将字符串转换成整数  若碰到非数字，停止
static int skip_atoi(const char **s)
{
	int i=0;

	while (is_digit(**s))
		i = i*10 + *((*s)++) - '0';
	return i;
}

// example:
// int main(void) {
// 	const char *s = "123a4";
// 	int a = skip_atoi(&s);
// 	printf("%d\n",a);
// 	return 0;
// }	
//  out: 123

#define ZEROPAD	1		/* pad with zero */
#define SIGN	2		/* unsigned/signed long */
#define PLUS	4		/* show plus */
#define SPACE	8		/* space if plus */
#define LEFT	16		/* left justified */
#define SPECIAL	32		/* 0x */
#define SMALL	64		/* use 'abcdef' instead of 'ABCDEF' */


// 除操作。输入：n 为被除数，base 为除数；结果：n 为商，函数返回值为余数
#define do_div(n,base) ({ \
int __res; \
__asm__("divl %4":"=a" (n),"=d" (__res):"0" (n),"1" (0),"r" (base)); \
__res; })

 // 将整数转换为指定进制的字符串。 
 // 输入：num-整数；base-进制；size-字符串长度；precision-数字长度(精度)；type-类型选项。 
 // 输出：str 字符串指针
static char * number(char * str, int num, int base, int size, int precision
	,int type)
{
	char c,sign,tmp[36];
	const char *digits="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
	int i;

	if (type&SMALL) digits="0123456789abcdefghijklmnopqrstuvwxyz";
	if (type&LEFT) type &= ~ZEROPAD;
	if (base<2 || base>36)
		return 0;
	c = (type & ZEROPAD) ? '0' : ' ' ;         // 如果类型指出要填零，则置字符变量 c='0'（也即''），否则 c 等于空格字符。
	if (type&SIGN && num<0) {
		sign='-';
		num = -num;
	} else
		sign=(type&PLUS) ? '+' : ((type&SPACE) ? ' ' : 0);
	if (sign) size--;
	if (type&SPECIAL)
		if (base==16) size -= 2;
		else if (base==8) size--;
	i=0;
	if (num==0)
		tmp[i++]='0';
	else while (num!=0)
		tmp[i++]=digits[do_div(num,base)];       //                                                 ===========key=============
	if (i>precision) precision=i;  //若数值字符个数大于精度值，则精度值扩展为数字个数值
	size -= precision;
	// 从这里真正开始形成所需要的转换结果，并暂时放在字符串 str 中
	if (!(type&(ZEROPAD+LEFT)))   // 若类型中没有填零(ZEROPAD)和左靠齐（左调整）标志，则在 str 中首先填放剩余宽度值指出的空格数。若需带符号位，则存入符号。
		while(size-->0)
			*str++ = ' ';
	if (sign)
		*str++ = sign;
	if (type&SPECIAL)             // 若类型指出是特殊转换，则对于八进制转换结果头一位放置一个'0'；而对于十六进制则存放'0x'
		if (base==8)
			*str++ = '0';
		else if (base==16) {
			*str++ = '0';
			*str++ = digits[33];
		}
	if (!(type&LEFT))
		while(size-->0)
			*str++ = c;
	while(i<precision--)
		*str++ = '0';
	while(i-->0)
		*str++ = tmp[i];      // 将转数值换好的数字字符填入 str 中。共 i 个, reverse                      ===========key=============
	while(size-->0)
		*str++ = ' ';
	return str;
}


//example
// int main(void) {
//     char *s = malloc(20);
//     char *s2;
//     s2 = number(s, 100, 16, 7, 5, 32);
//     for (int i = 0; i < 7; ++i) {
//         printf("%c",(s-7)[i]);
//     }
//     printf("\n");
//     return 0;
// }
// out: 0X00064        num = 100, base = 16, size = 7, precise = 5, type = 32


// 下面函数是送格式化输出到字符串中。 v for variable, s for string end with '\0'
int vsprintf(char *buf, const char *fmt, va_list args)
{
	int len;
	int i;
	char * str;            // 用于存放转换过程中的字符串
	char *s;
	int *ip;

	int flags;		/* flags to number() */    /* number()函数使用的标志 */

	int field_width;	/* width of output field */
	int precision;		/* min. # of digits for integers; max
				   number of chars for from string */
	int qualifier;		/* 'h', 'l', or 'L' for integer fields */

	for (str=buf ; *fmt ; ++fmt) {
		if (*fmt != '%') {
			*str++ = *fmt;
			continue;
		}
			
		/* process flags */
		flags = 0;
		repeat:
			++fmt;		/* this also skips first '%' */
			switch (*fmt) {
				case '-': flags |= LEFT; goto repeat;
				case '+': flags |= PLUS; goto repeat;
				case ' ': flags |= SPACE; goto repeat;
				case '#': flags |= SPECIAL; goto repeat;
				case '0': flags |= ZEROPAD; goto repeat;
				}
		
		/* get field width */
		field_width = -1;
		if (is_digit(*fmt))
			field_width = skip_atoi(&fmt);
		else if (*fmt == '*') {
			/* it's the next argument */     //float??
			field_width = va_arg(args, int);
			if (field_width < 0) {
				field_width = -field_width;
				flags |= LEFT;
			}
		}

		/* get the precision */
		precision = -1;
		if (*fmt == '.') {
			++fmt;	
			if (is_digit(*fmt))
				precision = skip_atoi(&fmt);
			else if (*fmt == '*') {
				/* it's the next argument */
				precision = va_arg(args, int);
			}
			if (precision < 0)
				precision = 0;
		}

		/* get the conversion qualifier */
		qualifier = -1;
		if (*fmt == 'h' || *fmt == 'l' || *fmt == 'L') {
			qualifier = *fmt;
			++fmt;
		}

		switch (*fmt) {
		case 'c':
			if (!(flags & LEFT))
				while (--field_width > 0)
					*str++ = ' ';
			*str++ = (unsigned char) va_arg(args, int);
			while (--field_width > 0)
				*str++ = ' ';
			break;

		case 's':
			s = va_arg(args, char *);
			len = strlen(s);
			if (precision < 0)
				precision = len;
			else if (len > precision)
				len = precision;

			if (!(flags & LEFT))
				while (len < field_width--)
					*str++ = ' ';
			for (i = 0; i < len; ++i)
				*str++ = *s++;
			while (len < field_width--)
				*str++ = ' ';
			break;

		case 'o':
			str = number(str, va_arg(args, unsigned long), 8,
				field_width, precision, flags);
			break;

		case 'p':
			if (field_width == -1) {
				field_width = 8;
				flags |= ZEROPAD;
			}
			str = number(str,
				(unsigned long) va_arg(args, void *), 16,
				field_width, precision, flags);
			break;

		case 'x':
			flags |= SMALL;
		case 'X':
			str = number(str, va_arg(args, unsigned long), 16,
				field_width, precision, flags);
			break;

		case 'd':
		case 'i':
			flags |= SIGN;
		case 'u':
			str = number(str, va_arg(args, unsigned long), 10,
				field_width, precision, flags);
			break;

		case 'n':
			ip = va_arg(args, int *);
			*ip = (str - buf);
			break;

		default:
			if (*fmt != '%')
				*str++ = '%';
			if (*fmt)
				*str++ = *fmt;
			else
				--fmt;
			break;
		}
	}
	*str = '\0';
	return str-buf;
}



// example:       the usage of %n
// int main()
// {
//     int check;
//     int a, b;
  
//     // Input two variables
//     scanf("%d%d%n", &a, &b, &check);
  
//     // Print value of a, b, and check
//     printf("%d\n%d\n%d", a, b, check);
//     return 0;
// }

// in: 24   56
// out: 24
// 	 56
// 	 7